Add a layout history which can be stepped through with select-layout -u

and -U commands (bound to 'u' and 'U' by default).

window.c

@@ -292,6 +292,7 @@ window_create1(u_int sx, u_int sy)
 
 	w->lastlayout = -1;
 	w->layout_root = NULL;
+	TAILQ_INIT(&w->layout_list);
 
 	w->sx = sx;
 	w->sy = sy;